1. Field of the Invention
The present invention relates to a device for generating a recording clock for recording to an optical disk, typically DVD-R or DVD-RW media, having wobbled recording grooves, and a method thereof.
2. Description of the Related Art
A typical method for generating the recording clock for recording to an optical disk having wobbled recording grooves such as those that are printed to DVD-R media is known, for example, from Japanese Patent Laid-Open Publication (kokai) H10-293926. The wobble signal is a continuous signal with a frequency component that is synchronized to disk rotation. The cited method uses this characteristic to generate a recording clock signal that is precisely synchronized to disk rotation by generating a clock signal phase that is synchronized to the continuous signal with a frequency multiplied PLL. The wobble signal is produced by detecting reflection from the groove by using a detector which is segmented in the tracking direction, passing the difference of the signals from the detector segments (a push-pull signal) through a bandpass filter, and digitizing the output of the bandpass filter.
As the recording density of newer optical disk media has increased, the track pitch has decreased to the point where crosstalk between adjacent groove tracks cannot be ignored. This is particularly a problem with DVD-R media, which is written with a CLV format, because if the wobble phase shifts in each rotation of the disk, the wobble phase will not match from track to track. Even more specifically, the wobble signal will be modulated by crosstalk between the tracks. Conventional optical discs of this type also have a prepit signal consisting of prepits that are formed at a specific interval to the lands (and therefore are called land prepits). The phase of this land prepit signal that is detected from the optical disk can therefore be compared with the phase of the wobble signal so as to output a phase difference signal, which is then used to correct the phase of the recording clock signal. In other words, variation in the time base of the clock signal based on a wobble signal containing track crosstalk that cannot be ignored is corrected by using a land prepit signal that is not affected by crosstalk so as to generate a recording clock signal that is synchronized with high precision to disk rotation.
Phase correction on DVD-R media is known to require a correction of ±30 degrees for a 360° wobble period. The recording clock for DVD-R media is obtained by multiplying the wobble signal by 186. It is therefore necessary to correct the phase of 186×(30/360)=16 clock periods in order to correct the phase of the recording clock. Because a stable phase correction for more than one recording clock period is difficult, a practical design such as the one disclosed in Kokai H10-293926 uses two phase-locked loops That is, the cited design uses a first PLL to output at the period of a relatively low frequency wobble signal, then shifts the phase of the first PLL output, and uses a second PLL to frequency multiply the shifted first PLL output so as to generate the recording clock.
A further problem is that if the wobble period fluctuates in an area where there are no land prepits, recording clock jitter increases. A method has also been proposed for resolving this problem by changing the response characteristic of the second PLL in the areas where there are and there are not land prepits so that the PLL response characteristic is lowered where there are no land prepits and recording clock jitter is thus improved.
Problems relating to generating the recording clock for an optical disk having a wobbled recording groove are described below.
First, as noted above, recording density has increased to the point where crosstalk between adjacent groove tracks cannot be ignored. With 4.7 GB DVD-R media, for example, the track pitch is 0.74 μm and the wobble period is 24.7 μm. The track pitch is thus smaller than that of first generation 3.9 GB DVD-R media, and the wobble signal phase varies periodically at the relatively slow frequency of about 5.3 revolutions. Phase correcting the recording clock generating PLL using the land prepit signal provides a certain improvement in recording clock jitter resulting from crosstalk, but the only method that has been proposed to address jitter in areas where there are no land prepits is to lower the PLL response. A method for handling land prepits detection errors has not been disclosed.
Furthermore, a practical phase shift circuit according to the prior art requires two PLL circuits, a PLL set to the wobble frequency of 140 kHz, and a 26 MHz frequency multiplying PLL for generating the recording clock. The problem with this configuration is the circuit scale increases. More particularly, digital chip processes are increasingly directed to smaller device dimensions with an emphasis on high speed, highly integrated circuit designs. The problem with a 140 kHz wobble frequency PLL is that the low frequency makes integration into modern digital chips difficult. If many analog PLL components such as charge pumps and VFO devices are built in to a digital chip, chip size increases, high precision noise management is required to assure the desired PLL jitter performance, and chip design is thus made more difficult. A further problem is the basic incompatibility between the analog phase shift circuit and a digital chip design. The analog phase shift circuit is usually used to shift the output phase of a 140 kHz wobble frequency PLL.
Second, while the wobble signal is generated by digitizing the push-pull signal output from the bandpass filter, the land prepit signal is also superimposed on the push-pull signal. The land prepit signal is not completely removed by the bandpass filter. More specifically, the wobble signal edge shifts when a land prepit is near the slice level for digitizing the wobble, and the wobble signal period is therefore not correct.
Third, the sensing level of the photodetector in the write head varies greatly according to laser power modulation for recording marks and spaces when recording to a disk. With DVD-R media, for example, laser modulation switches between 11 mW at a recording mark and 0.7 mW at a space, and there is therefore over a 10× variation in photodetector output between the marks and spaces. The wobble signal is extracted by obtaining a push-pull signal from the photodetector output and passing the result through a bandpass filter, and the effects of variations in recording power are therefore largely removed. However, some effect of power modulation remains in the push-pull signal and power modulation near the slice level for digitizing the wobble shifts the wobble signal edge so that the wobble signal period is not correct.
Fourth, the amplitude and DC level of the light that is sensed by the photodetector vary greatly during recording because the laser power changes from the read power level to the record power level. With DVD-R media, the read power level is typically 0.7 mW and the average record power level for marks and spaces during recording is approximately 7 mW or about ten times the read power level. If this difference is passed straight through the bandpass filter and digitized, the amplifier will be saturated during recording and the signal will be buried in noise during reproduction. An automatic gain control (AGC) circuit is therefore normally inserted to the RF amplifier so as to absorb this fluctuation, and feedback control is used to keep the signal amplitude constant before being digitized. However, correct digitizing cannot be expected during the transient states immediately after the start and end of recording, the wobble signal period will vary greatly, and wobble signal detection will be difficult for a time.
Fifth, while crosstalk effects are reduced by phase compensation based on the land prepits, the recording clock fluctuates when the land prepits are incorrectly detected or not detected. Yet further, land prepit detection also has a high frequency jitter component as a result of the recording power fluctuation discussed in the third problem above.
Finally, no specific solution for the second to fifth problems described above is known from the prior art.